How hard are you hitting it? Maybe you're not allowing it to wick completely. After a while, there's less juice for it to atomize. Give it a slight tip and let the wicks soak some juice up.
Did you flood the atomizer?
When I get less than usual hits, the first thing I do is unscrew my clearo and check to see if there's liquid lying on the battery. If that happens, dry it out, make sure to clear the center hole on your tank/clearo, and try again.
Is your battery running down? If all else fails, try a freshly charged battery and see if that improves it. Sometimes your battery will not function at peak even if it still has some charge left.
I'm sure there's other things to try, but those are the quick things to try.